Light guide and method for producing transparent thermoplastic resin composition for light guide

ABSTRACT

A light guiding plate, characterized by comprising a transparent thermoplastic resin composition containing 1-200 ppm of fine particles having a refractive index of 1.7-3.0 and an average particle diameter of 0.01-1.0 μm. This light guiding plate is suitable for display devices used in office automation apparatuses such as personal computers, word processors, etc. and various monitors displaying image signals such as panel monitors, television monitors, etc., display devices used in illuminators for indoor or outdoor space, and signs.

TECHNICAL FIELD

[0001] The present invention relates to a light guiding plate suitablefor display devices used in office automation apparatuses such aspersonal computers, word processors, etc., and various monitorsdisplaying image signals such as panel monitors, television monitors,etc., display devices used in illuminators for indoor or outdoor space,and signs.

BACKGROUND ART

[0002] Transparent thermoplastic resins, especially, methacrylic resinshave been employed for a wide variety of illumination uses because oftheir excellent light transmittance and mechanical characteristics.Recently, they are used as light guiding plates for back-lighting indisplay devices provided with illumination lamps. As the back-lightingsystems, there are generally used two systems of so-called direct-lighttype in which the light guiding plate is interposed between a lightsource and a liquid crystal unit, and edge-light type in which a lightsource is provided at the side edge portions of the light guiding plate,and at present the edge-light type is mainly employed. Especially, withrecent strong demands for high luminance display devices, large-sizeddisplay devices and thin display devices, luminous devices have beendeveloped under the conception of making lighter, larger and thinnerdevices, and, among them, high luminance face luminous devices ofedge-light type are especially desired.

[0003] Therefore, as to the light guiding plates used in light sourcedevices, there have also been strongly demanded light guiding platesaccording to which loss of incident light entering from light sourcelamp disposed at the side edge portion is reduced as much as possibleinside the light guiding plate and the incident light is allowed toefficiently outgo toward the outgoing face.

[0004] For meeting these demands, a plurality of technical disclosureshave been made on methods for attaining high luminance by using lightguiding plate. For example, JP-B-39-1194 discloses a method of obtaininga uniform luminous face by incorporating and dispersing light diffusingparticles in a substrate of the light guiding plate. Furthermore,JP-A-4-145485 discloses to attain a high luminance by using a lightscattering plastic material containing fine particles different inrefractive index as a light guide. Moreover, JP-A-2000-113708 disclosesa method for attaining high luminance by dispersing fine particleshaving hollow structure and differing in refractive index in a lightguiding plate.

[0005] In the above prior art, fine particles are incorporated, butoptimization of the kind and the average particle diameter of the fineparticles is not made, and the effect to improve luminance is small, andthus in case the display devices are made larger and thinner,sufficiently high luminance has not yet been obtained.

[0006] An object of the present invention is to provide a light guidingplate suitable for display devices used for office automationapparatuses such as personal computers, word processors, etc., andvarious monitors displaying image signals such as panel monitors,television monitors, etc., display devices used for illuminators forindoor or outdoor space, and signs.

[0007] Another object of the present invention is to provide a methodfor stable production of a resin composition for the light guidingplate.

DISCLOSURE OF INVENTION

[0008] As a result of intensive research conducted by the inventors forsolving the above problems, it has been found that a light guiding plateobtained from a transparent thermoplastic resin composition containing agiven amount of light scattering fine particles having a specificrefractive index and a specific average particle diameter can change theprogressing direction of the incident light entering from a light sourcelamp disposed along the side edge of the light guiding plate to adirection perpendicular to the luminous face of the light guiding plateand efficiently scatter the incident light to the side of the luminousface, whereby luminance of face luminescence can be enhanced. Thus, thepresent invention has been accomplished.

[0009] That is, the light guiding plate of the present inventioncomprises a transparent thermoplastic resin composition containing atransparent thermoplastic resin and fine particles, the refractive indexand the average particle diameter of the fine particles being 1.7-3.0and 0.01-1.0 μm, respectively, and the amount of the fine particlesbeing 1-200 ppm based on the weight of the transparent thermoplasticresin.

[0010] Furthermore, the method for producing the transparentthermoplastic resin composition constituting the light guiding plate ofthe present invention comprises previously dispersing fine particles inan organic liquid, thereby uniformly dispersing the fine particles inthe transparent thermoplastic resin.

BRIEF DESCRIPTION OF DRAWING

[0011]FIG. 1 shows one example of an edge-light type liquid crystallight source apparatus which uses the light guiding plate of the presentinvention, and FIG. 1A is a sectional view of the apparatus and FIG. 1Bis a top view of the apparatus. In the drawing, A indicates a lightsource (a cold cathode-ray tube), B indicates a lamp house, C indicatesa light guiding plate, D indicates a light reflective sheet, E indicatesa light diffusing sheet, and F indicates a prism sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] The present invention will be explained in detail below.

[0013] The light guiding plate of the present invention comprises atransparent thermoplastic resin composition containing a transparentthermoplastic resin and fine particles.

[0014] As the transparent thermoplastic resin contained in thetransparent thermoplastic resin composition, mention may be made ofmethacrylic resins, polycarbonate resins, styrene resins, cyclic olefinresins, amorphous polyesters, etc. Preferred are methacrylic resins,polycarbonate resins and cyclic olefin resins, and more preferred aremethacrylic resins.

[0015] As the methacrylic resins, there may be used copolymers of methylmethacrylate or ethyl methacrylate with a monomer copolymerizabletherewith. The amount of methyl methacrylate or ethyl methacrylate ispreferably not less than 70% by weight based on the weight of thecopolymer.

[0016] Examples of the monomer copolymerizable with methyl methacrylateor ethyl methacrylate are methacrylate esters such as butylmethacrylate, ethyl methacrylate, methyl methacrylate, propylmethacrylate, cyclohexyl methacrylate, phenyl methacrylate, 2-ethylhexylmethacrylate, etc.; acrylate esters such as methyl acrylate, ethylacrylate, butyl acrylate, cyclohexyl acrylate, phenyl acrylate,2-ethylhexyl acrylate, etc.; unsaturated acids such as methacrylic acid,acrylic acid, etc.; and the like. The monomers are not limited to theseexamples.

[0017] The method for the preparation of the methacrylic resins has nospecial limitation, and they can be prepared by conventional methods.

[0018] Furthermore, the methacrylic resins used in the present inventioninclude heat resistant methacrylic resins, low hygroscopic methacrylicresins, high-impact methacrylic resins, etc. The high-impact methacrylicresins are, for example, blends of methacrylic resins with rubberelastomers. The rubber elastomers are disclosed in JP-A-53-58554,JP-A-55-94917, JP-A-61-32346, etc.

[0019] As the polycarbonate resins, there may be used polymers derivedfrom divalent phenol compounds, a representative of which is bisphenolA. The method for the preparation of the polycarbonate resins has alsono particular limitation, and may be prepared by well known methods suchas phosgene method, ester interchange method, solid phase polymerizationmethod, etc.

[0020] As the cyclic olefin resins, there may be used amorphousthermoplastic resins such as polymers containing cyclic olefin skeletonsin the polymer chains such as norbornene, cyclohexadiene, etc. orcopolymers containing norbornene, cyclohexadiene, etc. The method forthe preparation of them has no special limitation.

[0021] For example, as the cyclic olefin resins mainly composed ofnorbornene, there may be used resins disclosed in JP-A-60-168708,JP-A-62-252406, JP-A-2-133413, JP-A-63-145324, JP-A-63-264626,JP-A-1-240517, JP-B-57-8815, etc. Furthermore, if necessary, thereto maybe added soft polymers such as olefinic soft polymers comprisingα-olefins, isobutylenic soft polymers comprising isobutylene, diene softpolymers comprising conjugated dienes such as butadiene, isoprene, etc.,cyclic olefin soft polymers comprising cyclic olefins such asnorbornene, cyclopentene, etc., organopolysiloxane soft polymers, softpolymers comprising α,β-unsaturated acids and derivatives thereof, softpolymers comprising unsaturated alcohols and amines or acyl derivativesthereof or acetals, polymers of epoxy compounds, fluororubbers, etc.

[0022] As the styrene resins, there may be used homopolymers orcopolymers containing styrene as an essential component, polymer blendsobtained from these polymers and other resins, etc. It is especiallypreferred to use polystyrene, AS resins which are copolymer resins ofacrylonitrile and styrene, and MS resins which are copolymer resins ofmethacrylate esters and styrene. Moreover, transparent reinforcedpolystyrenes having a styrene resin phase in which rubbers aredistributed can also be suitably used. The method for the preparation ofthe styrene resins has no special limitation and can be known methods.

[0023] As the amorphous polyesters, there may be used amorphous resinsamong polyesters formed from one or two or more dihydroxy compound unitsselected from aliphatic glycols such as ethylene glycol, propyleneglycol, 1,4-butanediol, neopentyl glycol, hexamethylene glycol, etc.,alicyclic glycols such as cyclohexanedimethanol, etc., and aromaticdihydroxy compounds such as bisphenol, 1,3-bis(2-hydroxyethoxy)benzene,1,4-bis(hydroxyethoxy)benzene, etc. and one or two or more dicarboxylicacid units selected from aromatic dicarboxylic acids such asterephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid,etc., aliphatic dicarboxylic acids such as oxalic acid, adipic acid,sebacic acid, succinic acid, undecadicarboxylic acid, etc., andalicyclic dicarboxylic acids such as hexahydroterephthalic acid, etc.The method for the preparation of the amorphous polyesters has nospecial limitation and can be known methods. Commercially easilyavailable ones are KODAR PETG or PCTA manufactured by Eastman Kodak Co.,Ltd., etc.

[0024] Next, the fine particles contained in the transparentthermoplastic resin composition will be explained.

[0025] The fine particles are present in the state of being dispersed inthe transparent thermoplastic resin and function as a light diffusingagent, and can improve the luminance of the light guiding plate when thetransparent thermoplastic resin composition containing the transparentthermoplastic resin and the fine particles is made into a light guidingplate.

[0026] The refractive index of the fine particles is 1.7-3.0, preferably1.7-2.5, more preferably 1.7-2.0. If the refractive index is less than1.7, the scattering properties are weak, and if it exceeds 3.0, thescattering properties are too strong, and when the progressing directionof light incident from a light source lamp disposed along the side edgeis changed to a direction perpendicular to the luminous face of thelight guiding plate, scattering in the vicinity of the lamp becomes toostrong, and as a result, unevenness in luminance and unevenness in colortone of outgoing light are apt to occur.

[0027] The refractive index in the present invention is a value based onthe D line (589 nm) at the temperature of 20° C. As a method for themeasurement of refractive index of fine particles, there is, forexample, a method according to which the fine particles are dipped in aliquid whose refractive index can be changed little by little andinterfaces of the fine particles are observed while changing therefractive index of the liquid, and the refractive index of the liquidwhen the interfaces of the fine particles become indefinite is measured.For the measurement of the refractive index of the liquid, Abberefractometer, etc. can be used.

[0028] The average particle diameter of the fine particles is 0.01-1.0μm. If the average particle diameter exceeds 1.0 μm, when theprogressing direction of light incident from the light source lampdisposed along the side edge is changed to a direction perpendicular tothe luminous face of the light guiding plate, loss of light occurs dueto back reflection, etc., and, hence, the incident light cannot beefficiently scattered toward the side of the luminous face and theobjective luminance of face luminescence can hardly be obtained. If theaverage particle diameter is less than 0.01 μm, it becomes difficult toscatter the incident light, and the objective luminance of faceluminescence cannot be obtained.

[0029] As the method for the measurement of average particle diameter ofthe fine particles, there are a method of dispersing the fine particlesin an organic liquid and measuring a 50% cumulative particle diameterusing a micro-track method, a method of obtaining a particle diameterfrom a transmission type electron photomicrograph of the fine particlesand employing the average value of the particle diameters as the averageparticle diameter, and the like.

[0030] Furthermore, the amount of the fine particles is 1-200 ppm basedon the weight of the transparent thermoplastic resin. Considering thebalance between luminance and unevenness in color tone and outgoinglight, the amount is preferably 3-100 ppm, more preferably 5-70 ppm. Ifthe amount of the fine particles is less than 1 ppm, substantially noeffect to improve the luminance is exhibited. If the amount of the fineparticles exceeds 200 ppm, in the light incident from the light sourcelamp disposed along the side edge, the proportion of the incident lightwhose progressing direction is changed to the direction perpendicular tothe luminous face in the vicinity of the lamp is too large and nosufficient light reaches the central part of the face luminous body inthe case of a large-sized liquid crystal display device of 15 inches ormore. As a result, even if a dot pattern is applied to the side of thelight guiding plate opposite to the luminous face for correcting theoutgoing light, the central part of the luminous face becomes dark andit becomes difficult to properly balance the unevenness of outgoinglight. If the proportion of the fine particles exceeds 200 ppm,yellowness of the light guiding plate increases, and, hence,distribution of color tone is apt to occur in the outgoing light withinthe luminous face.

[0031] The shape of the fine particles may be any of true sphere form,spherical form, flaky form, cubic form, indeterminate form, etc., and isnot particularly limited.

[0032] The impurities contained in the fine particles are desirablyreduced as much as possible, and purity of the fine particles is notless than 90% by weight, preferably not less than 95% by weight, morepreferably not less than 99% by weight.

[0033] The fine particles in the present invention have no speciallimitation as far as they satisfy the above conditions, and examplesthereof are aluminum trioxide (refractive index: 1.7-1.8), titaniumdioxide (refractive index: 2.5-2.8), etc. From the point of balancingbetween the effect to improve luminance of face luminescence and theunevenness in color tone within the luminous face, it is most preferredto use aluminum trioxide as the fine particles. Titanium dioxide isgreat in the effect to improve luminance of face luminescence, butsometimes causes difference in color tone of outgoing light in thevicinity of lamp and color tone of outgoing light in the central part.

[0034] The method for producing the transparent thermoplastic resincomposition has no special limitation as far as the fine particles areuniformly dispersed in the transparent thermoplastic resin.

[0035] However, preferably, the fine particles are previously uniformlydispersed in an organic liquid and the transparent thermoplastic resincomposition is produced using the resulting dispersion. That is, forproducing the transparent thermoplastic resin composition constitutingthe light guiding plate of the present invention, it is preferred touniformly disperse the fine particles in the transparent thermoplasticresin by previously dispersing the fine particles in an organic liquid.Furthermore, for uniformly dispersing the fine particles in the organicliquid, it is preferred to use an ultrasonic wave generating apparatus.

[0036] The organic liquids here include polymerizable monomersconstituting the transparent thermoplastic resin in addition to generalorganic liquids, and have no special limitation as far as the fineparticles are hardly dissolved therein and hardly swollen therewith, andthe fine particles can be uniformly dispersed therein. Moreover, severalkinds of the organic liquids can be mixed at optional proportions andused as a mixture depending on the dispersion state of the fineparticles.

[0037] Examples of the general organic liquids are ketones such asacetone, methyl ethyl ketone, etc., aromatics such as xylene, toluene,etc., and alcohols such as methanol, ethanol, etc. In the case of thetransparent thermoplastic resin being a methacrylic resin, examples ofthe polymerizable monomers are methacrylate esters such as methylmethacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, cyclohexyl methacrylate, phenyl methacrylate, 2-ethylhexylmethacrylate, etc., acrylate esters such as methyl acrylate, ethylacrylate, butyl acrylate, cyclohexyl acrylate, phenyl acrylate,2-ethylhexyl acrylate, etc., unsaturated acids such as methacrylic acid,acrylic acid, etc., and the like.

[0038] As methods for uniformly dispersing the fine particles in thetransparent thermoplastic resin in the production of the transparentthermoplastic resin composition containing the transparent thermoplasticresin and the fine particles, the following methods can be exemplified.

[0039] (1) When the transparent thermoplastic resin and the fineparticles are melt kneaded by an extruder:

[0040] There is a method which comprises dispersing the fine particlesin an organic liquid, preferably, using an ultrasonic wave generatingapparatus, mixing the resulting dispersion with the transparentthermoplastic resin, and melt kneading the mixture by an extruder. Inthis case, the organic liquid used has no limitation as far as the fineparticles are not dissolved therein and not swollen therewith, and thefine particles can be uniformly dispersed therein as mentioned above.Moreover, several kinds of the organic liquids can be mixed at optionalproportions and can be used as a mixture depending on the dispersionstate.

[0041] The mixing ratio of the fine particles and the organic liquid canbe optionally determined taking into consideration the dispersibility ofthe fine particles, but it is preferred that amount of the fineparticles is in the range of 0.001-80 parts by weight based on 100 partsby weight of the organic liquid.

[0042] The mixing ratio of the dispersion comprising the fine particlesand the organic liquid and the transparent thermoplastic resin can alsobe optionally determined taking into consideration the handleability atthe mixing extrusion step, but it is preferred that amount of thedispersion is in the range of 0.001-10 parts by weight based on 100parts by weight of the transparent thermoplastic resin.

[0043] The method for mixing the dispersion and the transparentthermoplastic resin has no special limitation. For example, there may beemployed known mixing methods such as mixing by Henschel mixer, mixingby super-floater and mixing by tumbler.

[0044] As to the extruders used for melt kneading of the mixture, thereis no need to use special extruders and they may be usual single-screwor twin-screw extruders, etc. However, from the point of removal ofvolatile components in the organic liquid used for dispersion, preferredare those which can perform devolatilization by reducing the pressure topreferably not higher than 300 Torr at a vent. Furthermore, from thepoint of inhibiting the secondary aggregation of the fine particles, thetwin-screw extruders are preferred for production of the composition.Temperature of the extruders can be optionally set depending on the kindof the transparent thermoplastic resin used. For example, it is about180-260° C. in the case of methacrylic resin.

[0045] (2) When polymerization is carried out by a casting method toobtain a sheet:

[0046] There is a method which comprises dispersing the fine particlesin a starting monomer for the transparent thermoplastic resin or amonomer copolymerizable with said starting monomer, preferably, using anultrasonic wave generating apparatus. In this case, it is preferred thatthe fine particles are previously dispersed in a part of the startingmonomer, followed by mixing with a partially polymerized polymersolution, or the like. The ratio of the amount of the fine particles andthat of the starting monomer in which the fine particles are dispersedcan be optionally determined considering dispersibility, viscosity atcharging, handleability, etc.

[0047] Furthermore, there are no special limitations in polymerizationconditions such as polymerization temperature, polymerization time,amount of the polymerization initiator, etc. in the casting method andin the method for the formation of sheet (cast plate). As a method forthe formation of sheet, mention may be made of, for example, glass cellcasting method, continuous casting method, etc.

[0048] The ultrasonic wave generation apparatus used for dispersion ofthe particles has no special limitation, and commercially availableultrasonic cleaning machines, ultrasonic stirrers, etc. can be used. Forexample, ultrasonic cleaning machines of 28-100 KHz in ultrasonicfrequency are generally used. The irradiation time by the ultrasonicwave generating apparatuses can be optionally set depending on thedispersion state of the fine particles, and is preferably 1-60 minutes.

[0049] In the present invention, if necessary, an ultraviolet absorbercan be added to the transparent thermoplastic resin composition whichconstitutes the light guiding plate. By the addition of the ultravioletabsorber, coloration due to ultraviolet rays generated from the lightsource lamp disposed along the side edge of the light guiding plate canbe prevented. Especially, in light source devices for color, even afterthe use of long period of time, color tone of the monitor picture planecan be always kept constant, occurrence of unevenness in color can beinhibited, and, besides, decrease of luminance and increase ofunevenness in luminance can also be inhibited. Examples of theultraviolet absorbers are benzotriazole-based ultraviolet absorbers suchas 2-(5-methyl-2-hydroxyphenyl)benzotriazole,2-[2-hydroxy-3,5-bis(α,α′-dimethylbenzyl)phenyl]benzotriazole,2-(3,5-di-t-amyl-2-hydroxyphenyl)benzotriazole, etc.; benzophenone-basedultraviolet absorbers such as 2-hydroxy-4-methoxybenzophenone,2,2′-dihydroxy-4-methoxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone,etc.; and salicylic acid-based ultraviolet absorbers such as phenylsalicylate, 4-t-butylphenyl salicylate, etc. These ultraviolet absorbersmay be used each alone or in combination of two or more.

[0050] The ultraviolet absorber can be added in a concentration of30-2000 ppm, preferably 80-500 ppm based on the transparentthermoplastic resin. If the concentration of the ultraviolet absorber isless than 30 ppm, the effect is small, and if it exceeds 2000 ppm,coloration of the light guiding plate due to the addition of theultraviolet absorber increases and, besides, decrease of luminance iscaused.

[0051] Moreover, in the light guiding plate of the present invention,the transparent thermoplastic resin composition may contain fatty acidesters of glycerin such as glycerin monostearate, etc., higher alcoholssuch as stearyl alcohol, or higher fatty acids such as stearic acid asreleasing agents, or antioxidants such as of phenolic type, thioethertype, phosphite type, etc. in such a range of concentration as nothindering the attainment of the object of the present invention(usually, in a concentration of not more than 5000 ppm).

[0052] The method for molding the light guiding plate of the presentinvention has no special limitation, and there may be employed knownmethods, for example, (1) a method of molding the transparentthermoplastic resin composition to a sheet by a sheet molding extruderor a press molding machine, cutting the resulting sheet to a desiredsize, and subjecting the cut surface to abrasive working, (2) a methodof molding the transparent thermoplastic resin composition by aninjection molding machine having a mold, and (3) a method of dispersingthe fine particles in a syrup containing a starting monomer for thetransparent thermoplastic resin or a partial polymer, then polymerizingthe monomer or the partial polymer by a casting method to obtain asheet-like molded article, then cutting it to a desired size, andsubjecting the cut surface to abrasive working. In case the lightguiding plate is obtained by molding the transparent thermoplastic resincomposition by a sheet molding extruder, a press molding machine, aninjection molding machine having a mold, etc., from operational andeconomical view points, there may be employed a method including thesteps of preparing a master batch pellet which has a higherconcentration of the fine particles in the thermoplastic resincomposition than the desired concentration and diluting to the desiredconcentration with transparent thermoplastic resin at the time ofmolding.

EXAMPLE

[0053] The present invention will be more specifically explained by thefollowing examples and comparative examples, which should not beconstrued as limiting the invention in any manner.

[0054] (Method for Measurement of Average Particle Diameter)

[0055] The average particle diameter of the fine particles was measuredby the following method.

[0056] As to the fine particles of not less than 0.1 μm in averageparticle diameter, the fine particles were dispersed in an organicliquid by an ultrasonic wave, the resulting dispersion was subjected tomeasurement by micro-tracking method, and a 50% cumulative particlediameter was taken as an average particle diameter.

[0057] As to the fine particles of less than 0.1 μm in average particlediameter, the fine particles were photographed by a transmission typeelectron microscope, and the longer diameter and the shorter diameter ofthe resulting particle image were measured. Assuming the average valuethereof to be a particle diameter of one particle, the average value ofparticle diameters of thirty fine particles was taken as an averageparticle diameter.

[0058] (Method for Measurement of Luminance of the Light Guiding Plate,and Method for Visual Evaluation of Unevenness in Color Tone andUnevenness in Outgoing Light)

[0059] The measurement and the evaluation were conducted by the lightsource apparatus shown in FIG. 1. Specifically, a cold cathode tube of 4mmφ (manufactured by Harison Electric Co., Ltd.) as the light source Awas provided in the lamp house B disposed at both side edges on the 319mm length sides of the light guiding plate C. REFWHITE RW 75CB(manufactured by Kimoto & Co., Ltd.) was used as the light reflectingsheet D, one sheet of D121 (manufactured by Tsujiden Co., Ltd.) as thelight diffusing sheet E was put on the upper surface of the lightguiding plate C, and two sheets of BEFII (manufactured by Sumitomo 3MCo., Ltd.) as the prism sheet F were put on the light diffusing sheet Ein such a manner that the prism rows of one sheet crossed those ofanother sheet at right angles (i.e., the prism rows of the lower prismsheet crossed the cold cathode-ray tube at right angles, and the prismrows of the upper prism sheet were parallel to the cold cathode tube). Avoltage of 12 V was applied to the cold cathode tube from a directvoltage stabilization apparatus, and after lighting for 20 minutes,luminance at 100 measuring points which were provided by dividing thewhole luminous face into 100 portions (10×10 in length and breadth) wasmeasured by a luminance meter (CA-1000 manufactured by Minolta CameraCo., Ltd.) provided at a distance of 1 m from the luminous face. Then,an average luminance of the resulting measured values was calculated.Furthermore, the luminous face was visually observed to evaluateunevenness of color tone and that of outgoing light.

[0060] (Method for Measurement of Light Transmittance of Long OpticalPath)

[0061] Tristimulus values XYZ of a luminous flux transmitting through atest piece of 220 mm length (6 mm in width×30 mm in height×220 mm inlength) were measured at an angle of visual field of 10° using a colorand color difference meter Model TC-1500 MC manufactured by TokyoDenshoku Co., Ltd. as a measuring device and standard light C as a lightsource. The value Y was taken as a light transmittance (Asahi Kaseimethod).

[0062] (Method for Measurement of Yellowness of Long Optical Path)

[0063] Tristimulus values XYZ of a luminous flux transmitting through atest piece of 220 mm length (6 mm in width×30 mm in height×220 mm inlength) were measured at an angle of visual field of 10° using the aboveTC-1500 MC as a measuring device and standard light C as a light source.Yellowness α of the test piece was calculated from the resulting valuesXYZ by the following formula (1). Furthermore, without setting the testpiece, tristimulus values XYZ were measured, and yellowness β of air wassimilarly calculated by the following formula (1). The resulting valuesα and β were substituted for the yellowness α and β in the followingformula (2) to obtain the long optical path yellowness of the respectivetest pieces.

Yellowness(α, β)=100 (1.28X−1.06Z)/Y  (1)

Long optical path yellowness=yellowness α−yellowness β  (2)

[0064] (Preparation of Methacrylic Resin Pellets)

[0065] 150 ppm of 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane and300 ppm of n-octylmercaptan were added to a monomer mixture comprising79.9% by weight of methyl methacrylate, 5.1% by weight of methylacrylate and 15% by weight of ethylbenzene and these were uniformlymixed. The resulting mixed solution was continuously fed to a closingtype pressure resistant reaction vessel of 10 liters in internal volume,and polymerization was carried out at an average temperature of 130° C.for an average residence time of 2 hours with stirring. Then, theresulting polymer was continuously delivered to a storage tank connectedto the reaction vessel, the volatile components were removed underreduced pressure, and the polymer was continuously transferred to anextruder in molten state. A given amount of2-(5-methyl-2-hydroxyphenyl)benzotriazole (an ultraviolet absorber)molten by heating at 140° C. was fed to the extruder from a side partthereof by a feed pump, and these were extruded to obtain a methacrylicresin pellets. The resulting methacrylic resin pellets were analyzed tofind that they had a copolymerization ratio of 94.0% by weight of methylmethacrylate unit and 6.0% by weight of methyl acrylate unit, andcontained 150 ppm of 2-(5-methyl-2-hydroxyphenyl)benzotriazole.

[0066] (Preparation of Raw Material Pellets A)

[0067] 0.06 g of aluminum trioxide (average particle diameter: 0.45 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of the methacrylic resin pellets, followed byblending the pellets as they were by a Henschel mixer (manufactured byMitsui Miike Kogyo Co., Ltd.) at 1400 revolutions for 1 minute. Thisoperation was repeated until the amount of the mixed pellets reached thenecessary amount. The resulting mixed pellets were extruded by a 30 mmφtwin-screw extruder (manufactured by Nakatani Co., Ltd.) while carryingout devolatilization under reduced pressure of 100 Torr to obtain amethacrylic resin composition containing 20 ppm of aluminum trioxide.This was referred to as raw material pellets A.

[0068] (Preparation of Raw Material Pellets B)

[0069] 0.60 g of aluminum trioxide (average particle diameter: 0.05 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of the methacrylic resin pellets, followed bycarrying out blending and extrusion in the same manner as in thepreparation of the raw material pellets A to obtain a methacrylic resincomposition containing 200 ppm of aluminum trioxide. This was referredto as raw material pellets B.

[0070] (Preparation of Raw Material Pellets C)

[0071] 0.021 g of aluminum trioxide (average particle diameter: 2 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid ofxylene:xylene:methanol=3:1 for 30 minutes at an oscillation frequency of38 KHz using an ultrasonic cleaning machine (US-4 manufactured byIUCHI), and it was confirmed that uniform dispersion was attained.Thereafter, the dispersion was uniformly sprayed on 3 kg of themethacrylic resin pellets, followed by carrying out blending andextrusion in the same manner as in the preparation of the raw materialpellets A to obtain a methacrylic resin composition containing 7 ppm ofaluminum trioxide. This was referred to as raw material pellets C.

[0072] (Preparation of Raw Material Pellets D)

[0073] 0.15 g of titanium dioxide (average particle diameter: 0.04 μm,refractive index: 2.71; manufactured by Ishihara Sangyo Co., Ltd.) waspre-dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of the methacrylic resin pellets, followed bycarrying out the same blending and extrusion as in the preparation ofthe raw material pellets A to obtain a methacrylic resin compositioncontaining 50 ppm of titanium dioxide. This was referred to as rawmaterial pellets D.

[0074] (Preparation of Raw Material Pellets E)

[0075] 0.15 g of aluminum trioxide fine particles (average particlediameter: 0.45 μm, refractive index: 1.76; manufactured by Nippon LightMetal Co., Ltd.) were uniformly sprayed on 3 kg of the methacrylic resinpellets without carrying out the pre-dispersion in the organic liquid,followed by blending the pellets as they were by a Henschel mixer(manufactured by Mitsui Miike Kogyo Co., Ltd.) at 1400 revolutions for 1minute. Thereafter, the pellets were extruded in the same manner as inthe preparation of the raw material pellets A to obtain a methacrylicresin composition containing 50 ppm of aluminum trioxide. This wasreferred to as raw material pellets E.

[0076] (Preparation of Raw Material Pellets F)

[0077] 0.15 g of calcium carbonate (average particle diameter: 0.04 μm,refractive index: 1.66) was dispersed in 20 g of a mixed organic liquidof xylene:methanol=3:1 for 30 minutes at an oscillation frequency of 38KHz using an ultrasonic cleaning machine (US-4 manufactured by IUCHI),and it was confirmed that uniform dispersion was attained. Thereafter,the dispersion was uniformly sprayed on 3 kg of the methacrylic resinpellets, followed by carrying out the same blending and extrusion as inthe preparation of the raw material pellets A to obtain a methacrylicresin composition containing 50 ppm of calcium carbonate. This wasreferred to as raw material pellets F.

[0078] (Preparation of Raw Material Pellets G)

[0079] 0.15 g of barium sulfate (average particle diameter: 0.06 μm,refractive index: 1.64) was dispersed in 20 g of a mixed organic liquidof xylene:methanol=3:1 for 30 minutes at an oscillation frequency of 38KHz using an ultrasonic cleaning machine (US-4 manufactured by IUCHI),and it was confirmed that uniform dispersion was attained. Thereafter,the dispersion was uniformly sprayed on 3 kg of the methacrylic resinpellets, followed by carrying out the same blending and extrusion as inthe preparation of the raw material pellets A to obtain a methacrylicresin composition containing 50 ppm of barium sulfate. This was referredto as raw material pellets G.

[0080] (Preparation of Raw Material Pellets H)

[0081] 0.30 g of aluminum trioxide (average particle diameter: 0.45 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of the methacrylic resin pellets, followed bycarrying out the same blending and extrusion as in the preparation ofthe raw material pellets A to obtain a methacrylic resin compositioncontaining 100 ppm of aluminum trioxide. This was referred to as rawmaterial pellets H.

Example 1

[0082] The raw material pellets A and the methacrylic resin pellets wereuniformly mixed at a weight ratio of 1:19 by a tumbler. The resultingmixed pellets were extruded at a temperature of 250° C. using anextrusion sheet molding machine comprising a 50 mmφ single-screwextruder having a T-die for sheet, a temperature-controlled polishingroll and a take-off device to obtain an extruded plate of 300 mm inwidth and 6 mm in thickness containing 1 ppm of aluminum trioxide. Then,from the resulting extruded plate, a plate of 241 mm in width and 319 mmin length was cut out using a circular saw. The cut surface of thisplate was subjected to abrasion by a precision abrading machine(PLA-BEAUTY manufactured by Megaro Technica Co., Ltd.) and furthersubjected to buffing to perform mirror finishing, thereby obtaining alight guiding plate containing 1 ppm of aluminum trioxide.

Comparative Example 1

[0083] A light guiding plate was produced in the same manner as inExample 1, except that only the methacrylic resin pellets were used andthe raw material pellets A were not used.

Example 2

[0084] A light guiding plate containing 7 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that the mixingweight ratio of the raw material pellets A and the methacrylic resinpellets was changed to 7:13.

Example 3

[0085] A light guiding plate containing 20 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that only the rawmaterial pellets A were used without carrying out the dilution with themethacrylic resin pellets.

Examples 4 and 5

[0086] A light guiding plate containing 50 ppm or 100 ppm of aluminumtrioxide was produced in the same manner as in Example 1, except thatthe raw material pellets B were used and the mixing weight ratio of theraw material pellets B and the methacrylic resin pellets was 1:3 or 1:1.

Example 6

[0087] A light guiding plate containing 200 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that only the rawmaterial pellets B were used without carrying out the dilution with themethacrylic resin pellets.

Example 7

[0088] The raw material pellets A and the methacrylic resin pellets wereuniformly mixed at a weight ratio of 7:13 by a tumbler to obtain a rawmaterial for injection molding. The resulting mixed pellets were moldedat a temperature of 260° C. using an injection molding machine (IS550manufactured by Toshiba Machine Co., Ltd.) having a mold and a moldtemperature controlling device to obtain a light guiding plate of 241 mmin width, 319 mm in length and 6 mm in thickness which contained 7 ppmof aluminum trioxide.

Example 8

[0089] 0.014 g of aluminum trioxide (average particle diameter: 0.45 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was added to 100 g of methyl methacrylate, and a dispersion was preparedby dispersing the particles using an ultrasonic cleaning machine (US-4manufactured by IUCHI) for 30 minutes at an oscillation frequency of 38KHz. A methacrylic resin syrup was prepared by dissolving 400 g ofpolymethyl methacrylate having an average molecular weight of 100,000 in1500 g of methyl methacrylate. To the syrup were added the abovedispersion, 0.8 g of 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile) and0.3 g of 2-(5-methyl-2-hydroxyphenyl)benzotriazole as an ultravioletabsorber, followed by mixing uniformly while carrying out vacuumdeaeration. The resulting mixture was poured into a cell comprising twoglass plates of 9 mm in thickness and a gasket in accordance withconventional cell casting method, the cell was sealed, thenpolymerization was carried out for 12 hours in a water bath controlledto 40° C. in temperature, and thereafter post-polymerization was carriedout at 110° C. for 3 hours to obtain a cast plate of 6 mm in thicknesswhich contained 7 ppm of aluminum trioxide and 150 ppm of2-(5-methyl-2-hydroxyphenyl)benzotriazole. Then, from the resulting castplate, a plate of 241 mm in width and 319 mm in length was cut out usinga circular saw. The cut surface of this plate was subjected to abrasionby a precision abrading machine (PLA-BEAUTY manufactured by MegaroTechnica Co., Ltd.) and further subjected to buffing to perform mirrorfinishing, thereby obtaining a light guiding plate containing 7 ppm ofaluminum trioxide.

Example 9

[0090] A light guiding plate containing 7 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that the rawmaterial pellets E were used and the mixing weight ratio of the rawmaterial pellets E and the methacrylic resin pellets was 7:43.

Comparative Example 2

[0091] A light guiding plate containing 0.5 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that the rawmaterial pellets A were used and the mixing weight ratio of the rawmaterial pellets A and the methacrylic resin pellets was changed to1:39.

Comparative Example 3

[0092] 0.75 g of aluminum trioxide (average particle diameter: 0.05 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of the methacrylic resin pellets, followed bycarrying out blending and extrusion in the same manner as in thepreparation of the raw material pellets A to obtain a methacrylic resincomposition containing 250 ppm of aluminum trioxide. A light guidingplate containing 250 ppm of aluminum trioxide was produced in the samemanner as in Example 1, except that only the resulting compositionpellets were used without carrying out the dilution with the methacrylicresin pellets.

Comparative Example 4

[0093] A light guiding plate containing 7 ppm of aluminum trioxide wasproduced in the same manner as in Example 1, except that only the rawmaterial pellets C were used without carrying out the dilution with themethacrylic resin pellets.

Comparative Example 5

[0094] A light guiding plate containing 50 ppm of calcium carbonate wasproduced in the same manner as in Example 1, except that only the rawmaterial pellets F were used without carrying out the dilution with themethacrylic resin pellets.

Comparative Example 6

[0095] A light guiding plate containing 50 ppm of barium sulfate wasproduced in the same manner as in Example 1, except that only the rawmaterial pellets G were used without carrying out the dilution with themethacrylic resin pellets.

Example 10

[0096] 0.15 g of aluminum trioxide (average particle diameter: 0.05 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of polycarbonate resin pellets(PANLITE/L-1250Y manufactured by Teijin Chemicals Ltd.), followed byblending the pellets as they were by a Henschel mixer (manufactured byMitsui Miike Kogyo Co., Ltd.) at 1400 revolutions for 1 minute. Thisoperation was repeated until the amount of the mixed pellets reached thenecessary amount. The resulting mixed pellets were extruded by a 30 mmφtwin-screw extruder (manufactured by Nakatani Co., Ltd.) at 260° C.while carrying out devolatilization at the vent part under reducedpressure of not higher than 100 Torr to obtain a polycarbonate resincomposition containing 50 ppm of aluminum trioxide.

[0097] The resulting polycarbonate resin composition, the polycarbonateresin pellets and 2-(5-methyl-2-hydroxyphenyl)benzotriazole as anultraviolet absorber were uniformly mixed at a weight ratio of1:6.14:0.0011 by a tumbler. The resulting mixed pellets were extruded ata temperature of 260° C. using an extrusion sheet molding machinecomprising a 50 mmφ single-screw extruder having a T-die for sheet, atemperature-controlled polishing roll and a take-off device to obtain anextruded plate of 300 mm in width and 6 mm in thickness which contained7 ppm of aluminum trioxide and 150 ppm of2-(5-methyl-2-hydroxyphenyl)benzotriazole. Then, a light guiding platecontaining 7 ppm of aluminum trioxide was obtained in the same manner asin Example 1.

Example 11

[0098] 0.15 g of aluminum trioxide (average particle diameter: 0.05 μm,refractive index: 1.76; manufactured by Nippon Light Metal Co., Ltd.)was dispersed in 20 g of a mixed organic liquid of xylene:methanol=3:1for 30 minutes at an oscillation frequency of 38 KHz using an ultrasoniccleaning machine (US-4 manufactured by IUCHI), and it was confirmed thatuniform dispersion was attained. Thereafter, the dispersion wasuniformly sprayed on 3 kg of a cyclic olefin resin pellets (ZEONOR 1060Rmanufactured by Nippon Zeon Co., Ltd.), followed by blending the pelletsas they were by a Henschel mixer (manufactured by Mitsui Miike KogyoCo., Ltd.) at 1400 revolutions for 1 minute. This operation was repeateduntil the amount of the mixed pellets reached the necessary amount. Theresulting mixed pellets were extruded by a 30 mmφ twin-screw extruder(manufactured by Nakatani Co., Ltd.) at 250° C. while carrying outpurging the hopper with nitrogen and devolatilization at the vent partwith reducing the pressure to not higher than 100 Torr to obtain acyclic olefin resin composition containing 50 ppm of aluminum trioxide.

[0099] The resulting cyclic olefin resin composition, the cyclic olefinresin pellets and 2-(5-methyl-2-hydroxyphenyl)benzotriazole as anultraviolet absorber were uniformly mixed at a weight ratio of1:6.14:0.0011 by a tumbler. The resulting mixed pellets were extruded ata temperature of 250° C. using an extrusion sheet molding machinecomprising a 50 mmφ single-screw extruder having a T-die for sheet, atemperature-controlled polishing roll and a take-off device whilepurging the hopper part with nitrogen to obtain an extruded plate of 300mm in width and 6 mm in thickness which contained 7 ppm of aluminumtrioxide and 150 ppm of 2-(5-methyl-2-hydroxyphenyl)benzotriazole. Then,a light guiding plate containing 7 ppm of aluminum trioxide was obtainedin the same manner as in Example 1.

Examples 12 and 13

[0100] A light guiding plate containing 7 ppm or 20 ppm of titaniumdioxide was produced in the same manner as in Example 1, except that theraw material pellets D were used and the mixing weight ratio of the rawmaterial pellets D and the methacrylic resin pellets was 7:43 or 2:3.

[0101] Luminance of the light guiding plates obtained in Examples 1-13and Comparative Examples 1-6 was measured, and visual evaluation onunevenness of color tone and unevenness of outgoing light was conducted.The results are shown in Table 1.

[0102] The light guiding plates of Examples 1-13 had considerably higherface luminous performance as compared with the light guiding plates ofComparative Examples 1, 2, 4, 5 and 6. The light guiding plate ofComparative Example 3 was somewhat improved in average luminance, butwas conspicuous in the phenomenon that the outgoing light in thevicinity of lamp was great while the outgoing light in the centralportion was too small (uneven outgoing light) and, besides, conspicuousin the phenomenon that it was strong in blueness in the vicinity of lampand strong in yellowness in the central portion (uneven color tone), andthus the light guiding plate was unsuitable as a light guiding plate fordisplay devices such as liquid crystal monitors.

[0103] Furthermore, the light guiding plates of Examples 5, 6 and 13showed some phenomena of uneven color tone and uneven outgoing light,but they were usable as light guiding plates for display devices such asliquid crystal monitors.

Example 14

[0104] One surface of the light guiding plate obtained in Example 2 wassubjected to screen printing using a printing screen of 15 inches insize having a dot gradation and using matte medium SR931 (manufacturedby Mino Group Co., Ltd.) as an ink, and another surface was providedwith a diffusion layer. Luminance was measured in the same manner as inExample 1. The light guiding plate had an average luminance of 3100cd/M², and did not show uneven color tone and uneven outgoing light.Thus, it had excellent performances.

Comparative Example 7

[0105] The light guiding plate obtained in Comparative Example 1 wassubjected to printing in the same manner as in Example 14, and luminancewas measured. The light guiding plate had an average luminance of 2700cd/m², and did not show uneven color tone and uneven outgoing light, butwas lower 400 cd/m² in average luminance than the light guiding plate ofExample 14.

Example 15

[0106] A test piece having a plate thickness of 6 mm, an optical pathlength of 220 mm and a height of 30 mm for measurement of lighttransmittance of long optical path and yellowness of long optical pathwas prepared by cutting out from the light guiding plate obtained inExample 2 using a circular saw and subjecting both cut surfaces throughwhich light entered to abrasion by a precision abrading machine(PLA-BEAUTY manufactured by Megaro Technica Co., Ltd.) and then tobuffing. Light transmittance of long optical path and yellowness of longoptical path were measured.

Examples 16-22

[0107] Light guiding plates containing 12 ppm,17 ppm, 22 ppm, 32 ppm, 42ppm, 70 ppm, and 100 ppm of aluminum trioxide were produced in the samemanner as in Example 1, except that the raw material pellets H were usedand the mixing weight ratio of the raw material pellets H and themethacrylic resin pellets was 12:88, 17:83, 22:78, 32:68, 42:58, 70:30,and 100:0. From the resulting light guiding plates, test pieces formeasurement of light transmittance of long optical path and yellownessof long optical path were prepared in the same manner as in Example 15,and light transmittance of long optical path and yellowness of longoptical path were measured.

Comparative Example 8

[0108] From the light guiding plate obtained in Comparative Example 1, atest piece for measurement of light transmittance of long optical pathand yellowness of long optical path was prepared in the same manner asin Example 15, and light transmittance of long optical path andyellowness of long optical path were measured.

[0109] The results of measurement in Examples 15-22 and ComparativeExample 8 are shown in Table 2.

[0110] The optical path length can be optionally set, but in Examples15-22 and Comparative Example 8, it was set longer, namely, at 220 mm,so that difference in the measured values can readily result due to thedifference in content of the fine particles. This optical path length of220 mm is a distance which corresponds to the distance between the lampdisposed at longer side edge of the light guiding plate and the centralportion of the light guiding plate in a liquid crystal monitor of, forexample, about 29 inches in size (the size ratio is shorter side:longerside=3:4).

[0111] Referring to the results of measurement in Examples 15-22, thelight transmittance of long optical path decreased with increase of thecontent of aluminum trioxide. This is because light scattering becamestrong due to the increase of content of fine particles, and theluminous flux entering from the lamp was scattered in and emerged fromthe test piece. If the light transmittance of long optical path is toolow, this causes “uneven outgoing light”, namely, the central portion ofthe light guiding plate is darker than the portion near the lamp.Moreover, with increase of the content of aluminum trioxide, theyellowness of the long optical path tends to increase. If the yellownessof the long optical path is too high, this causes “uneven color tone”,namely, the central portion of the light guiding plate becomesyellowish.

[0112] Here, it is natural that with change of the size in inch of theliquid crystal monitor, the optical path length also changes. Forexample, when the light guiding plate of the present invention is usedfor a liquid crystal monitor of 15 inches in size, the optical pathlength to the central portion of the light guiding plate is about 114mm, and when the light guiding plate of the present invention is usedfor a liquid crystal monitor of 10 inches in size, the optical pathlength to the central portion of the light guiding plate is about 76 mm.With change of the optical path length, the light transmittance and theyellowness naturally change, and there is a tendency that with decreaseof the optical path length, the light transmittance increases and theyellowness decreases.

[0113] Therefore, even in the case of a light guiding plate having alight transmittance of long optical path of less than 5% and ayellowness of long optical path of higher than 60 when the optical pathlength was 220 mm as in Examples 20-22, increase of luminance which isan effect of the present invention can be attained in liquid crystalmonitors of different size in inch. In other words, luminance of lightguiding plate is increased by properly adjusting the amount of the fineparticles having a specific refractive index and a specific averageparticle diameter in the range of the present invention depending on thesize of display devices, etc. in which the light guiding plate is used.

[0114] Furthermore, from the comparison of Example 15 in which was usedthe light guiding plate obtained in Example 2 with Comparative Example 8in which was used the light guiding plate obtained in ComparativeExample 1 (where fine particles were not contained in the resincomposition), it was recognized that the light transmittance of longoptical path in Example 15 was lower than that in Comparative Example 8,and the yellowness of long optical path in Example 15 was higher thanthat in Comparative Example 8. It can be said that this shows theabove-mentioned tendency that with increase of the content of aluminumtrioxide, the light transmittance of long optical path decreases and theyellowness of long optical path increases. However, in fact, unevennessin color tone and unevenness in outgoing light were also not seen in thelight guiding plate of Example 15. Moreover, as is clear from the aboveresults, the light guiding plate of Example 14 obtained by using thelight guiding plate of Example 2 as in Example 15 was superior 400 cd/M²in average luminance as compared with the light guiding plate ofComparative Example 7 obtained by using the light guiding plate ofComparative Example 1 (where fine particles were not contained in theresin composition). Thus, the light guiding plates of the presentinvention where a specific amount of fine particles are contained in theresin composition have very high face luminous performance irrespectiveof the size of display devices. TABLE 1 Visual evaluation on unevennessParticles of color Particle Method for Average tone and diameter AmountThermoplastic producing face luminance outgoing Kind (μm) (ppm) resinluminous body (cd/m²) light Example 1 Aluminum trioxide 0.45 1Methacrylic resin Extrusion method 80 No Example 2 Aluminum trioxide0.45 7 Methacrylic resin Extrusion method 1090 No Example 3 Aluminumtrioxide 0.45 20 Methacrylic resin Extrusion method 1330 No Example 4Aluminum trioxide 0.05 50 Methacrylic resin Extrusion method 1350 SlightExample 5 Aluminum trioxide 0.05 100 Methacrylic resin Extrusion method1400 Some Example 6 Aluminum trioxide 0.05 200 Methacrylic resinExtrusion method 1230 Some Example 7 Aluminum trioxide 0.45 7Methacrylic resin Injection molding 1085 No method Example 8 Aluminumtrioxide 0.45 7 Methacrylic resin Casting 1095 No Example 9 Aluminumtrioxide 0.45 7 Methacrylic resin Extrusion method 760 No Example 10Aluminum trioxide 0.45 7 Polycarbonate Extrusion method 995 No Example11 Aluminum trioxide 0.45 7 Cyclic olefin Extrusion method 1025 NoExample 12 Titanium dioxide 0.04 7 Methacrylic resin Extrusion method1370 Slight Example 13 Titanium dioxide 0.04 20 Methacrylic resinExtrusion method 1670 Some Comp. No addition — — Methacrylic resinExtrusion method 70 No Example 1 Comp. Aluminum trioxide 0.45 0.5Methacrylic resin Extrusion method 75 No Example 2 comp. Aluminumtrioxide 0.05 250 Methacrylic resin Extrusion method 500 Great Example 3Comp. Aluminum trioxide 2 7 Methacrylic resin Extrusion method 170 NoExample 4 Comp. Calcium carbonate 0.04 50 Methacrylic resin Extrusionmethod 75 No Example 5 Comp. Barium sulfate 0.05 50 Methacrylic resinExtrusion method 80 No Example 6

[0115] TABLE 2 Light transmit- tance of Yellowness long of longParticles optical optical Particle path path diameter AmountThermoplastic Production (220 mm (220 mm Kind (μm) (ppm) resin methodlength) length) Example 15 Aluminum trioxide 0.45 7 Methacrylic resinExtrusion method 40% 30 Example 16 Aluminum trioxide 0.45 12 Methacrylicresin Extrusion method 30% 38 Example 17 Aluminum trioxide 0.45 17Methacrylic resin Extrusion method 21% 44 Example 18 Aluminum trioxide0.45 22 Methacrylic resin Extrusion method 13% 51 Example 19 Aluminumtrioxide 0.45 32 Methacrylic resin Extrusion method  8% 53 Example 20Aluminum trioxide 0.45 42 Methacrylic resin Extrusion method  3% 61Example 21 Aluminum trioxide 0.45 70 Methacrylic resin Extrusion method 1% 67 Example 22 Aluminum trioxide 0.45 100 Methacrylic resin Extrusionmethod 0.6%  76 Comp. No addition — — Methacrylic resin Extrusion method90% 2 Example 8

INDUSTRIAL APPLICABILITY

[0116] The light guiding plates of the present invention are improved attheir maximum in face luminous efficiency of the plates per se, therebyattaining the enhancement of luminance, and, as a result, there areprovided light guiding plates suitable for display devices used inoffice automation apparatuses such as personal computers, wordprocessors, etc., and various monitors displaying image signals such aspanel monitors, television monitors, etc., display devices used inilluminators for indoor or outdoor space, and signs.

1. A light guiding plate which comprises a transparent thermoplasticresin composition containing a transparent thermoplastic resin and fineparticles, the refractive index and the average particle diameter of thefine particles being 1.7-3.0 and 0.01-1.0 μm, respectively, and theamount of the fine particles being 1-200 ppm based on the weight of thetransparent thermoplastic resin.
 2. A light guiding plate according toclaim 1, wherein the fine particles comprise aluminum trioxide.
 3. Alight guiding plate according to claim 1 or 2, wherein the transparentthermoplastic resin is a resin selected from the group consisting ofmethacrylic resins, polycarbonate resins and cyclic olefin resins.
 4. Alight guiding plate according to any one of claims 1-3, wherein thetransparent thermoplastic resin composition additionally contains anultraviolet absorber.
 5. A method for producing a transparentthermoplastic resin composition for a light guiding plate whichcomprises uniformly dispersing fine particles in a transparentthermoplastic resin, wherein the fine particles are previously dispersedin an organic liquid.
 6. A method for producing a transparentthermoplastic resin composition for a light guiding plate according toclaim 5, wherein the fine particles are dispersed in the organic liquidusing an ultrasonic wave generating apparatus.